This invention relates to spectroscopy, and specifically to external reflectance spectroscopy. It is concerned primarily with a form of reflectance spectroscopy in which chemical structure information about a material is determined directly by monitoring the Fresnel reflectance of the first surface of a dielectric material. It can also be used for what is often called reflection/absorption spectroscopy, in which the analysis is actually accomplished by absorption spectroscopy of a thin layer on a reflecting substrate.
Diffuse reflectance spectroscopy is widely used in the visible and near-infrared regions of the spectrum to study solid materials in their unmodified state. For example, visible diffuse reflectance is used for color analysis; and near-infrared diffuse reflectance is used for studying the physical and chemical properties of grains and other food products.
Diffuse reflectance spectroscopy takes advantage of the fact that most materials contain scattering centers (structural discontinuities) which tend to scatter light in a more or less random fashion. After multiple interactions with such centers, a significant fraction of the light will eventually be scattered back out of the sample. The amplitude of this signal will be dependent on the density of scattering centers, and on any absorption of the radiation by the sample material. By collecting this radiation a quantitative spectrum of the sample is obtained which is somewhat analogous to a conventional absorbance spectrum.
One desirable goal in external reflectance spectroscopy is extending its usefulness to the mid-infrared region. In principle, mid infrared spectroscopy has some marked advantages over visible and near-infrared spectroscopy for the analysis of organic materials. This is due to the fact that the fundamental absorbances of virtually all organic materials lie in the mid-infrared. These materials have very few absorption bands of interest in the visible region. The overtone bands which occur in the near-infrared tend to be broad and overlapping, making positive identification of species extremely difficult. In the mid-infrared region, the absorption bands are generally narrow and distinct, allowing absorbance subtraction and other techniques to be used to distinguish the spectral contributions of various species.
Despite its advantages, mid-infrared has not been widely used for the analysis of intact bulk samples. A primary reason for this is the fact that the mid-infrared properties of most materials are not advantageous for the use of diffuse reflectance. The effectiveness of the scattering centers in diffuse reflectance is inversely proportional to wavelength, so that scattering in the mid-infrared region tends to be relatively weak. At the same time, the fundamental absorbances which occur in this region are very strong. As a result, the signal levels obtained in mid-infrared diffuse reflectance of intact objects are often very weak, In addition, the spectra obtained are often distorted by the presence of first surface (dielectric) reflectance contributions. In the laboratory, this problem is often overcome by grinding the sample and mixing it with a nonabsorbing, scattering substance, such as potassium bromide powder. This approach is not suitable for industrial quality control applications, in which modification of the sample is undesirable.
Attempts have been made to obtain usable mid-infrared spectra of intact solids by such techniques as (a) photo acoustic spectroscopy (PAS), and (b) the use of an integrating sphere to maximize the diffuse signal collected. Neither of these approaches has proved generally acceptable. Integrating spheres tend to produce very low signal levels in the mid-infrared, while PAS is plagued by excess noise and spectral distortions.
An alternative approach to obtaining chemical information about intact samples is to take advantage of the fact that the refractive index, and hence the specular reflectance from the surface, of an organic substance are highly wavelength dependent. The resultant specular reflectance spectra can be used in an manner analogous to absorbance spectra to provide detailed chemical analysis.
The potential utility of specular reflectance spectroscopy (SRS) has been illustrated in recent publications, with particular emphasis on the fact that mathematical expressions called the "Kramers-Kronig relations" can be used to convert measured reflectance spectra into absorbance spectra. These can then be used to identify samples by comparison with existing libraries of absorbance spectra. However, the work reported has not attempted to use SRS for quantitative measurements such as the composition analysis of mixtures of organic substances.
Most of the measurements reported were performed with devices which make use of radiation of unknown polarization state, typically striking the sample with a range of incidence angles other than normal incidence. However, the form of the Kramers-Kronig relations used to perform the data conversion assumes that the radiation is at normal incidence. Thus, the calculated absorbance spectra obtained from such data can be expected to contain spurious angle and polarization dependent artifacts. Indeed such artifacts seem to be evident in some of the reported spectra.
In a paper presented at the FACSS Conference in October, 1989 by Doyle and McIntosh (Paper 424, 16th Annual FACSS Conference), it was concluded that the Kramers-Kronig relations could not be used to obtain accurate absorbance spectra from reflectance data unless the equations used were modified to take into consideration polarization and angle of incidence, or unless the experimental apparatus provided radiation which approximated the conditions at normal incidence.